Aluminum alloyed billets are widely used for extrusion due to their very good extrudability, resistance to corrosion, and potential for precipitation hardening by heat treatment. The most common alloying elements are magnesium and silicon. As-cast billets, however, require homogenization in order to obtain the desired properties for extrusion.
A complete homogenization cycle of the billets comprises heating, homogenization or soaking, and cooling. Each individual component of the cycle is critical. Soaking of the billets needs special attention, and subsequent cooling is particularly sensitive. Soaking is taking place at a temperature range defined below the solidus line of the alloy and above the solvus temperature of the various solid phases.
Following the soaking step is the important cooling step, wherein the cooling rate of the billets is crucial, since the distribution and state of the alloying elements magnesium and silicon are influenced by the rate of cooling of the billets. Depending on the distribution and state of magnesium and silicon in the aluminum alloy, the surface quality, the extrusion rate and the extrusion load will vary, and the mechanical properties of parts obtained from the alloy may greatly differ. The optimum cooling rate is determined by the alloy composition desired. For instance, in the AA-6XXX series of aluminum alloys, high homogenization temperatures shift the Mg.sub.2 Si precipitation curve to the right, thus making the alloy less quench sensitive, but transition elements like manganese, chromium, iron or zirconium move the precipitation curve to the left, and increase the quench sensitivity of the alloys. Consequently, it is a common practice in the art to add small amounts of manganese or chromium in the alloy, and quickly cool the billets from soak temperature to maximize the resistance of the balanced alloys.
The conventional manner to cool billets batchwise is to load them on a car or platform which is then placed in a cooling chamber wherein the billets are cooled by passing air therethrough with the help of exhaust fans suck. Although the air flow drawn by the fans is coming in the cooling chamber at a constant rate from the open side of the chamber, the billets at the centre of the load will obviously not be cooled at the same rate as those sitting close to the open side, since the temperature of the air increases as it progresses through the billets. The maximum variations in the cooling rate must therefore be maintained within a certain range, otherwise, as stated above, discrepancies will exist in the properties of the billets for a given load. The temperature difference between the front and back planes of the load is determined by the spacing between each row of billets, the fan capacity, the load capacity and the load configuration. On the other hand, the flow path of cooling air in the chamber depends on the load and the cooler geometry. In the event that the load is not symmetrical, significant variations of cooling rates would occur depending on the location of the billets in the car, as discussed in Computer Software in Chemical and Extractive Metallurgy, 1993, 319-331. The paper also suggests the use of baffles to optimize the air distribution during cooling. Although the mathematical model used and the plant data show that the baffles may be helpful in terms of optimizing the air flow, the reference is silent on the manner to lay the baffles above the billets, or the location, number or size of the baffles.
It would therefore be highly desirable to develop an adjustable baffling system to be installed in any conventional cooling chamber for providing a uniform cooling of billets, irrespective of the diameter of the billets or the configuration of the load, by uniformly dispersing air in the chamber.